The present invention relates to a control device for a copier of the type operable in a two-sided copy mode and, more particularly, to a sheet positioning control device which allows sheets to be fed to and refed from an intermediate tray to be accurately positioned on the tray with no regard to their size.
A copier operable of the type described is extensively used today and has an intermediate tray or two-side tray disposed on a sheet transport path which is provided in the copier. Paper sheets each carrying an image on one side thereof, or one-sided sheets, are sequentially stacked on the intermediate tray and then refed from the tray to an image transferring station to form other images on the other side thereof. A pair of side fences and a back fence are provided on the intermediate tray and movable in matching relation to the size of sheets for positioning the individual sheets on the tray. Specifically, the side fences are reciprocatingly movalbe toward and away from each other over a predetermined width to position the sheets on the tray. At the time of refeed, the side fences are shifted to and held at their predetermined position according to the sheet size so as to serve as a guide. The back fence is movable forward or backward to a predetermined position also matching the sheet size in an intended direction of sheet transport. The back fence maintains the leading edge of the sheet stack on the tray at a predetermined position, whatever the sheet size may be. The side fences and the back fence are driven by exclusive drive sources such as pulse motors which are controlled independently of each other. Specifically, a CPU or similar controller feeds a particular number of pulses assigned to each sheet size beforehand to each pulse motor, whereby each fence is moved by a predetermined distance. On the start of a two-sided copy mode operation and before a one-sided paper sheet enters the intermediate tray, the side fences are once moved to their home position and then to a predetermined position matching the size of sheets which will enter the tray. This is also true with the back fence. In this manner, both the side fences and the back fence are moved away from the individual home positions on the basis of particular numbers of pulses assigned to each sheet size. A home position sensor is located at each of the home positions for determining whether or not the associated fence is located in the home position. Usually, such home position sensors are initially adjusted in position at the time of assembly or shipment of the copier by use of a jig or like implement and then fixed in place by, for example, paint locking. The numbers of pulses are, therefore, determined with reference to the fixed positions of the home position sensors, and the fences are each driven to move a distance associated with the number of pulses.
However, while the copier is actually operated by the user, it sometimes occurs that the size of one-sided sheets entering the intermediate tray is different from their prescribed size. Specifically, the prescribed width of size A3 or similar comparatively large size is 420 millimeters, and the number of pulses is determined accordingly. In practice, however, the actual width of a one-sided sheet of size A3 is often smaller than the prescribed width by about 1.5 to 2.5 millimeters due to shrinkage which is caused by heat at an image fixing station also included in the copier. Then, even if the individual fences are moved on the basis of the predetermined numbers of pulses as with sheets of prescribed size, they fail to position the shrunken sheets accurately on the intermediate tray since the change in width is not compensated for at all. Should the paper sheets be not accurately positioned on the intermediate tray, they would undergo skew or similar undersirable occurence at the time of refeed to the image transferring station. It has been customary, therefore, that a serviceman readjusts the home positions of the individual fences, i.e., the positions of the home position sensors. This, however, renders the maintenance troublesome.